(Star Wars: Assault Team) How to get high-tier heroes

WP - "Star Wars: Assault Team features heroes of five different tiers. There are the one-tier, which are surprisingly uncommon and really really weak, and the two-tier heroes, which are slightly less weak, and very common. Things really start getting good with three-tier heroes, though, as they are quite a bit stronger; however, they are more rare. Four and five-tier heroes are the most powerful in the game, and consequently, the rarest."